The present invention relates to a substrate coated with a copper film in which the copper film is formed on the surface thereof by PVD (physical vapor deposition) process, and the copper film has an excellent corrosion resistance and the crystal oriented in the (111) face is preferentially grown. The present invention also relates to a method of controlling the crystal orientation in the (111) face of the copper film and forming the copper film.
Copper films formed on a substrate have been widely used as the electrode materials of IC-mounted boards and film capacitors. For forming the copper film on the substrate, PVD process, such as vacuum deposition and sputtering, has been normally used.
In forming the copper film, it is desirable to crystalize the copper film, in order to secure low electric resistance that is essential to the electrode material and high corrosion resistance against etching solution and environment. In the copper film, the corrosion resistance of the copper film is higher with increase of the crystal oriented in the (111) face in which copper atoms are arranged at the highest density. Therefore, it is preferable to grow copper crystal oriented in the (111) face as greatly as possible.
However, much crystallization of a copper film cannot be achieved by simply forming the copper film on the substrate by vacuum deposition process and the like. The crystal orientation in the copper film depends largely on a state of the surface of the substrate (e.g., crystal structure, irregularity, and wettability of the surface). Because of this, it is difficult to grow preferentially the crystal oriented in an intended crystal face (here, (111) face) alone. Therefore, the resultant copper film formed has insufficient corrosion resistance against etching solution and environment.
For this reason, the film formation is conventionally performed while heating the substrate at a desired temperature, so as to make it easy to grow the crystal in an intended crystal face. However, this method cannot be used for the substrate (e.g., plastic and organic films) of the type which has a low degree of heat tolerance. When this type of substrate is heated to high temperature, the substrate is distorted. Accordingly, it is impossible to preferentially grow the crystal oriented in the intended crystal face (in this instance, the (111) face) irrespective of the material of substrate.
An X-ray diffraction pattern of a prior copper film is shown in FIG. 2. The copper film was formed on the surface of a polyimide film by vacuum deposition process at room temperature without surface treatment. As seen, the X-ray diffraction intensity has peaks at the diffraction angles representing the copper crystal faces of (111), (200), and (220). This fact teaches that the copper film has been polycrystallized. The crystal oriented in the (111) face is small, and the X-ray diffraction intensity ratio thereat is 0.9 (cps/nm). The copper film of such small crystal oriented in that crystal face is insufficient in corrosion resistance. In forming an electric circuit pattern on the surface of the substrate, the copper-film coated substrate is immersed in an etching solution of hydrochloric acid series. At this time, pin holes are formed in the surface of the copper film. Etching solution penetrates through the pin holes into the boundary between the substrate and the copper film layer. As a result, close adhesion of the copper film to the substrate is remarkably reduced.